and it's working - but this question relates to calendar invites that have not been responded to that are sitting in a user's INBOX. i notice they are not being archived, nor are the attachments within them being archived. and hence taking up substantial mailbox space

The pending request will have message class like "IPM.Schedule.Meeting.request", so you might want to add this class in Directory properties in admin console and then select it in relevant mailbox policy and sync the users.